1. Field of the Invention
This subject invention pertains generally to the field of imaging devices, and in particular to those devices containing a sensor array. The invention is most particularly applicable to an X-ray image sensor array wherein high energy radiation is converted by a phosphor converter to visible light that is sensed by the sensor array for real time radiation imaging.
2. Background Art
Amorphous silicon two-dimensional sensor arrays are well-known devices for real time imaging of incident high energy radiation (see R. A. Street et al., "Amorphous Silicon Arrays Develop a Medical Image", IEEE Circuits and Devices, July 1993, pp. 38-42, for a general description of the structure of the arrays). Such sensor arrays are particularly advantageous for radiation imaging because they present a relatively large size image sensor array. Sensor arrays operate on the principal of integrating a charge representative of the quantities of visible light incident on the sensor. A phosphor converter-generates the visible light from incident high energy radiation. Such phosphor converters are well-known and generally operate by absorbing X-ray photons to produce high energy electrons which, in turn, generate electron hole pairs, which in turn form visible light when the electrons and holes recombine. It can thus be appreciated that the amount of visible light generated in the phosphor converter is directly related to the radiation incident on the phosphor.
In the x-ray imaging applications of amorphous silicon 2-d sensor arrays, a phosphor is placed in contact with the array surface, and light emitted by the phosphor is collected by the array. A high collection efficiency of the emitted light is a critical factor in the performance quality of the imager. Lowering the collection efficiency reduces the signal, and reduces the detective quantum efficiency (DQE), both because the quantum statistics are degraded by incomplete capture of the light and because the readout electronic noise is a larger fraction of the signals. Loss of DQE seriously affects the viability of a medical imaging product, because it directly affects patient x-ray dose.
A principal objective of any sensor array is thus to have a high light collection efficiency. To the extent that any light is lost or not sensed by the sensor array, loss of resolution and inefficiency in representation of the incident radiation occurs.
With reference to FIG. 1, the sensor array of an imager that is collecting the visible light is made up of a plurality of individual pixels 10, which collectively will represent the image generated by the imager. The light is generated in the phosphor converter 30 from radiation incident thereon. Each pixel typically contains an individual light sensor 12, a transistor 14 that functions as a switch to communicate a signal representative of the sensed light, and various metallization lines that allow the representative image to be read out to an external device. The metallization lines comprise gate lines 16 running in one direction that attach the sensor gate to the transistor, a plurality of data lines 18 running orthogonally to the gate lines for communicating the representative signal out to the external electronic device and a bias connection 20 to the sensor 12. As can be seen in FIG. 1, each of the silicon sensors is spaced from an adjacent sensor to accommodate the various metallization lines.
The sensor acts as a charge integrator. As light falls on it, it charges up and it continues to charge up until the transistor is switched on. In other words, the sensor operates in a first phase where it merely accumulates a charge representative of the amount of visible light incident on it, and then operates in a second "readout" phase when a pulse is applied to the gate line which turns on all the gates of those sensors in a column to which the particular pulse gate line is attached. The pulse causes the charge on the sensors to be transferred to the data lines for that column of sensors. Such an output essentially resets the sensors in the column back to zero so that they can then start the collecting of charge again.
Conventionally, once one column has been read out, the next column is immediately pulsed so that the entire array is sequentially read out from one side to the other.
As noted above, due to the requirement of the metallization lines to communicate the collected charge, the individual sensors are spaced so the sensor space which is available to collect visible light cannot cover the whole area of the pixel. The industry describes the percentage of area of the sensor array actually consumed by sensors as the "fill factor". It is desirable, of course, to have as high a fill factor as possible because fill factor is directly representative of the efficiency of the sensor array to collect the available visible light. Unfortunately, it is the nature of the technology illustrated in FIG. 1, that the fill factor can never be one hundred percent as the result of the necessity of the metallization communication lines.
Additionally, it is desirable to reduce the size of the pixel to increase the resolution of the image. As a consequence of this design goal, the metallization lines will consume a higher percentage of a space relative to a reduced size sensor so the fill factor will get accordingly smaller and smaller, and overall device sensitivity will decrease. Keeping in mind that only the fraction of the area of the array that exposes a sensor is capable of receiving light, if the fill factor is 25%, then the maximum fraction of light received is 25% and approximately 75% of the visible light generated by the phosphor converter is lost.
The present invention contemplates a new and improved device which overcomes the problem of lost light to provide a new high light efficiency collection X-ray image sensor array despite a low sensor fill factor. The invention is simple in design, economical to manufacture, readily adaptable to a plurality of particular sensor configuration and provides increased sensitivity of detection. Most importantly, the invention has the consequence of minimizing the necessary radiation dosage in medical imaging applications.